Some fixed-price projects allow you to propose milestones (payment for the project divided into predefined chunks rather than as a lump sum at the end). You can set up these milestones as part of the initial proposal or during the interview phase (before the creation of a contract). Use this option to set more specific or complex terms than a single bid can express.
Once a contract is created, all future milestone creation and updates must be done by the client.
Note that clients are only required to fund one milestone at a time. A milestone is not covered by Fixed-Price Protection until it is fully funded to escrow by the client.